After acute myocardial infarction (AMI), the damaged heart tissue starts a complex series of processes aimed to repair, and eventually to replace, the lesion by scar tissue [1].
During this period, the infarcted area is highly active biologically. A rapid turnover of cells and of structural components contributes to create new extracellular matrix (ECM). Preexisting ECM proteins (e.g. collagen) are digested by metalloproteinases (MMPs) and new matrix is laid down. Contextually, new cell migration and differentiation contribute to myocardial recovery after injury [1,2].
Starting from the earliest phases of wound repair, attempted by the extensive changes of tissue architecture, the vulnerable wound is exposed to the mechanical stress of the continuous heart beating cycles. This is, besides, responsible for drastic changes of the intraventricular pressure and volume. During the healing processes, myocardial contraction cannot be avoided, so the lesion cannot ever heal properly, if compared to other tissue lesions in which the mechanical stress is extremely lower and they can also be contained by provisional rigid scaffold/tutors (i.e. skin lesions, bone fractures, etc).
Although all the specific reparative processes are potentially appropriate and well-timed, very often they fail in avoiding adverse remodelling and loss of myocardial performances due to non-optimal scar formation at the injury site or even to scar rupture.
The first step (attempt) in the reparative processes after wound occurs is the formation of a three-dimensional fibrin meshwork aimed to block lesion expansion and furnish a provisional scaffold/platform for endogenous neo-vessels formation, cell recruiting and spreading. The earlier the process starts, the better the healing is due to prompt scar formation. In addition, proper healing steps should not be interrupted or hampered by enzymatic and/or mechanical actions (e.g. fibrin meshwork rupture due to unrestrained proteolytic activity or heart beating), specially in the earliest healing phases.
Normally, a high strength together with an extraordinary extensibility and elasticity of the fibrin net is warranted by the action of a circulating transglutaminase, factor XIII (FXIII), which by means of covalent cross-bonds links fibrin chains and several other ECM components changing drastically their original properties [3]. This fact lets the fibrin-scaffold to work properly by counteracting lesion expansion and furnishing the best setting and timing to have an optimal reparative process. Finally, cell migration/differentiation and neo-vessel formation are FXIII-dependent processes and strongly contribute to myocardial healing and recovery after injury.
FXIII is a pro-enzyme of plasma transglutaminase family, consisting of two enzymatic A subunits (FXIII-A) and two non-catalytic B subunits (FXIII-B) [3]. It plays a critical role in generation of a stable haemostatic plug, wound healing, tissue repair, and angiogenesis. FXIII is present in plasma, platelets, monocytes, and macrophages, all components deeply involved in infarct healing. Its key role on healing is demonstrated by the following:
(1) delayed haemorrhage in congenital FXIII deficient subjects;
(2) delayed wound healing in FXIII-deficient cases by human and animal models;
(3) positive effects by FXIII topical application on wound healing by in vivo and in vitro studies;
(4) antiapoptotic and proangiogenic properties;
(5) effects on cell migration/differentiation into the wound;
(6) modulation of fibrin and new collagen synthesis and deposition in ECM;
(7) strong positive effects in heart transplanted animals.
Extraordinary direct evidences of the essential role of FXIII in acute and chronic infarct scar stability come from an experimental model with genetically reduced FXIII levels in animals. Nahrendorf and coll. In 2006 [4], in mice deficient in (FXIII−/−), or heterozygous for (FXIII−/+) the FXIII-A gene (FXIII levels 5% and 70% respectively), by means of coronary ligation and high-field cardiac MRI, followed myocardial scar formation and the consequent remodelling process of the heart. Authors found that all and FXIII−/− mice died within 5 days after MI due to left ventricular rupture. On the contrary, FXIII−/− mice that received five days of intravenous FXIII replacement therapy had normal survival rates; though, their cardiac MRI demonstrated worse left ventricular remodelling. Again, by using a FXIII-sensitive molecular imaging probe, they found significantly greater FXIII activity in wild-type mice and in reconstituted FXIII−/− mice than in non-reconstituted FXIII−/− mice. Contextually, neutrophil cell migration into the MI scar was diminished in FXIII−/− mice but not in reconstituted FXIII−/− mice, and the physiological MMP-9 increasing, normally observed after MI, was 650% higher [4]. in non-reconstituted FXIII−/− mice. This, together with the observation that collagen-1 level was 53% lower in FXIII−/− mice, demonstrate an imbalance in ECM turnover and provides a possible explanations for the observed cardiac rupture in 100% of the FXIII deficient mice. In confirmation of the latter, mice KO for MMP-9 gene were protected against cardiac rupture and they survived. Thus, prevention of unrestrained MMP-9 upregulation by FXIII efficiently contributes to protection against post-MI cardiac rupture.
After the publication of Nahrendorf [4], several other papers reported data in favour of the extraordinary role of FXIII in the post-MI healing fate. Most of them dealt with poor healing and adverse remodelling as the primary cause of heart failure [5,6]. Other suggested FXIII as “The cement of the heart after myocardial infarction” [7], able to contrast infarct expansion and suggesting FXIII as a useful replacement therapy in patients with ventricular rupture [5].
The direct pro-healing properties of FXIII in post-infarcted heart. (i.e angiogenesis and cell proliferation/differentiation) and those mediated by the FXIII-stabilized matrix into the wound (i.e. modulation of fibrin and new collagen synthesis/deposition) prompted researchers even to propose intra-myocardial injections of polymeric biomaterial components or utilization of fibrin-cell-seeded scaffold in the first phases of MI. This could help in reducing infarct size and facilitating stem cell or growth factors delivery improving in turn better heart healing and higher survival [8,9].
Finally, the recently recognized role of the cellular immune response as essential in myocardial healing, ascribe to neutrophils, and monocytes/macrophages key functions in post-MI healing [10-13]. Briefly, in the first hours after ischemia, neutrophils accumulate in the infarcted myocardium and peak about within 24 hours; thereafter, monocytes/macrophages invade the lesion. Divergent and complementary functions have been associated to different monocyte subsets during the complex healing phases. In the inflammatory phase, neutrophils, and monocytes migrate in the infarcted myocardium to remove dead cells and debris and promote ECM degradation by activated MMPs; subsequently, monocytes/macrophages produce cytokines to repress inflammatory signals and regulate the formation of granulation tissue, essential for the proliferative phase. Now, new blood vessels, fibroblast proliferation, and ECM formation, favourite the maturation phase in which this tissue is substituted by a mature collagen based scar.
FXIII takes part to all of these functions, being at the intersection of coagulation, inflammation as well as wound healing [3]. In fact, recruitment of both macrophages and neutrophils are reduced in the infarcted heart of FXIII−/− KO mice as well as phagocytic activity. FXIII is also contained in these particular inflammatory cells, and it basically furnishes a more robust tri-dimensional meshwork with augmented elastic and extensible properties. This facilitates new vessel formation and cell differentiation avoiding imbalance in ECM turnover and attenuating inflammatory response to injury.
All the data described above are in favour that having appropriate levels of FXIII at the injury site is essential requisite for optimal wound healing particularly in the earliest phases. Accordingly, recent reports strongly suggest the needing to explore new treatment strategies to repair the injured heart, by augmentation of intrinsic wound healing that occurs during the first 1 to 2 weeks after MI. This also because on one hand the existence of efficient acute care (angioplasty, thrombolyses) have reduced drastically acute infarct mortality, and on the other the inadequate options to treat the increased number of infarct survivors has contributed to the growth of post-MI chronic complications in particular heart failure [1].
Accordingly, other studies suggest that a prompt FXIII supplementation could help heart in healing itself, because of the lack/deficiency of this enzyme invariably leads to the worst-prognosis of healing after myocardial injury [5,6,14]. Although intervening at this step is considered a promising, but underexploited, useful time window between acute reperfusion efforts and therapy against anomalous cardiac remodelling/chronic heart failure, one must be cautious and keep in mind that FXIII stabilizes blood clots (thrombus) which presence in coronaries is just responsible for occlusion/infarction [4].
The quality of infarct healing shortly after myocardial injury marks the fate of the patient for years to come. Nowadays, there no exist dedicated laboratory tests to efficiently predict post-MI healing outcome and/or to have prognostic information to help clinicians in earlier applying personalized treatment to avert anomalous ventricular remodelling and heart failure or other major adverse cardiac events (MACE).